A poor man . . . . . . . . I knew nursed the orphan.

Here is a breakdown of each option:

• A. who is used to refer to people. In this case, the poor man is the subject of the sentence, so we use “who” to refer to him.
• B. whom is used to refer to people as objects of verbs or prepositions. In this case, the poor man is the subject of the sentence, not the object of a verb or preposition, so we cannot use “whom”.
• C. that is used to refer to things. In this case, the poor man is a person, not a thing, so we cannot use “that”.
• D. which is also used to refer to things. In this case, the poor man is a person, not a thing, so we cannot use “which”.